Field of the Invention
The present invention relates to a processing method of an optical device wafer in which the optical device wafer obtained by forming a light emitting layer on a surface of a sapphire substrate and forming an optical device in each of plural regions marked out by plural planned dividing lines in a lattice manner is divided into individual optical device chips along the planned dividing lines.
Description of the Related Art
In an optical device manufacturing process, an optical device wafer is configured by stacking a light emitting layer (epitaxial layer) composed of an n-type gallium nitride semiconductor layer and a p-type gallium nitride semiconductor layer on a surface of a sapphire substrate having a substantially circular disc shape and forming optical devices such as light emitting diodes and laser diodes in plural regions marked out by plural planned dividing lines formed in a lattice manner. Then, the regions in which the optical devices are formed are separated by cutting the optical device wafer along the planned dividing lines to manufacture the individual optical device chips.
The above-described cutting of the optical device wafer along the planned dividing lines is normally carried out by cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table that holds a processing-target object, cutting means for cutting the processing-target object held by this chuck table, and cutting feed means that moves the chuck table and the cutting means relatively. The cutting means includes a spindle, a cutting blade mounted to this spindle, and a drive mechanism that rotationally drives the spindle. The cutting blade is composed of a base having a circular disc shape and a ring-shaped cutting edge mounted to the outer circumferential part of the side surface of this base. The cutting edge is obtained by fixing e.g. diamond abrasive grains with a grain size of approximately 3 μm to the base by electro-forming and is formed to a thickness of approximately 20 μm.
However, the sapphire substrate forming the optical device wafer has high Mohs hardness and the cutting by the cutting blade is not necessarily easy. Furthermore, because the cutting blade has a thickness of approximately 20 μm, the width of the planned dividing lines that mark out the devices needs to be approximately 50 μm. For this reason, there is a problem that the area ratio occupied by the planned dividing lines is high and the productivity is low.
To solve the above-described problem, the following method has been proposed as a method for dividing an optical device wafer along planned dividing lines. Specifically, laser-processed grooves serving as the start point of breaking are formed by irradiating a sapphire substrate with a pulse laser beam having such a wavelength as to be absorbed by the sapphire substrate along the planned dividing lines. Then, the wafer is split by giving an external force along the planned dividing lines along which the laser-processed grooves serving as the start point of breaking are formed (for example, refer to Japanese Patent Laid-Open No. 1998-305420).
However, there is the following problem when the laser-processed grooves are formed through irradiation with a laser beam along the planned dividing lines formed on a surface of the sapphire substrate forming the optical device wafer. Specifically, the outer circumferences of the optical devices such as light emitting diodes are subjected to ablation and a melted object called debris adheres to the optical devices. Thus, the luminance decreases and the quality of the optical devices is lowered.
To solve such a problem, the following processing method has been disclosed in Japanese Patent No. 3408805. Specifically, modified layers are formed inside a sapphire substrate on which a light emitting layer (epitaxial layer) is not formed along planned dividing lines by irradiating the sapphire substrate with a laser beam having such a wavelength as to be transmitted through the sapphire substrate along the planned dividing lines from the back surface side of the sapphire substrate, with the light focus point of the laser beam positioned inside the sapphire substrate. Thereby, the sapphire substrate is divided along the planned dividing lines along which the strength is lowered due to the formation of the modified layers.
However, when the modified layers are formed inside the sapphire substrate along the planned dividing lines, there are problems that the side surfaces of the optical devices are covered by the modified layers and the flexural strength or die strength of the optical devices is lowered and that it is impossible to vertically divide the sapphire substrate over the range from the back surface to the front surface.
To solve such a problem, the following laser processing method has been disclosed in Japanese Patent Laid-Open No. 2014-221483. Specifically, the numerical aperture (NA) of a condensing lens to condense a pulse laser beam is so set that a value obtained by dividing the numerical aperture (NA) of the condensing lens by the refractive index (N) of a single-crystal substrate is in a range of 0.05 to 0.2. Then, the single-crystal substrate is irradiated with a pulse laser beam condensed by this condensing lens and fine pores and amorphous regions that shield the fine pores are grown between the light focus point positioned in the single-crystal substrate and the incidence side of the pulse laser beam to form shielded tunnels.